The present invention relates generally to printed circuit boards (PCBs) and, more particularly, to methods and apparatus for suppressing radio frequency interference (RFI) or electromagnetic interference (EMI) generated by replaceable components on the printed circuit boards.
Currently, in order to remain competitive in the electronics industry, PCB designers must continuously reduce design time and corresponding cost. Thus, one trend is towards designing the PCB to include replaceable modular components. By using replaceable modular components, a designer may then upgrade and replace components on a PCB as new versions of components are made available. This trend is especially prevalent in the computer industry, where computer systems may be designed to include replaceable components.
FIG. 1 is a simplified, exploded perspective view of a conventional computer system 100. The computer system 100 includes a main logic board 102, a computer case 120, a fan housing 150 a hard drive 140, a power supply (not shown), and a floppy drive and CD-ROM drive 130. The brain of the computer system is the main logic board 102, commonly referred to as a central processing unit (CPU) board or "motherboard." The motherboard is basically a special PCB that includes the electronic circuitry that actually makes up the computer.
For example, the motherboard 102 may include a plurality of memory connectors 104 for receiving memory modules 106, a plurality of option connectors 112 for receiving option cards (not shown), a plurality of power connectors 118 for receiving power, an application specific integrated circuit (ASIC), a connector 108 for receiving a CPU module 110, and a fan housing 119.
Many of the components and associated connectors of the motherboard 102 may be configured to be upgradeable. For example, the connector 108 for the CPU module 110 may be designed to enable replacement of the CPU module 110. As shown, the CPU module 110 may then be inserted perpendicularly into the connector 108 on the motherboard 102. Thus, when a newer, faster CPU is designed, one may remove the older, slower CPU module and replace it with the newer, faster CPU module.
Although conventionally designed motherboards that include replaceable components have many advantages, these motherboards also present a number of problems. For example, as designers increase the operating frequency of individual components (e.g., CPU's), it becomes more difficult to meet the Federal Communication Commission's (FCC) requirements for devices that may emit radio frequency interference (RFI) or electromagnetic interference (EMI).
This RFI/EMI effect is especially prevalent in electronics products that operate at high frequencies. For example, a CPU that operates at 300 MHz may have a detrimental RFI/EMI effect at a third harmonic (e.g., three times the base frequency) of 900 MHz. Coincidentally, telephones operate at about 900 MHz, and thus a 300 MHz CPU may interfere with telephone communication if the computer system is not designed to attenuate RFI/EMI at 900 MHz.
Consequently, the FCC has strict guidelines that require electronics products to eliminate or substantially reduce RFI/EMI effects such that the operation of a particular electronics product will not substantially interfere with other communication devices (e.g., telephones and radios). Specifically, the energy emitted at certain proscribed frequencies by high frequency devices must be attenuated to levels that are within FCC limits. The details of the FCC guidelines are complex, and thus, will not be further described. However, the guidelines are laid out in Part 15 of the FCC Rules, which document is herein incorporated by reference in its entirety.
In order to comply with these stringent FCC requirements, systems designers typically add a network of bypass capacitors to the motherboard so as to bypass a range of high frequencies. For example, a network of bypass capacitors are typically added to the back surface of the motherboard and are connected between a ground plane and a power plane of the motherboard 102. These capacitors are configured to attenuate RFI/EMI for a limited range of frequencies. Typically, this range of frequencies will only include frequencies emitted from one type of component. That is, the capacitors are configured to suppress RFI/EMI for one type of component which operates at one particular frequency or a relatively small range of frequencies.
FIG. 2 is a simplified cut away top view of a portion of a motherboard 202 that includes the conventional power plane 215. The power plane 215 is typically used for routing power to power supply pins of various components on the mother board 202. In this example, the power plane 215 is electrically coupled to a particular component's interconnect holes 211 on the motherboard 202. That is, when the particular component is mounted onto the motherboard 202, a power supply pin of the particular component is then electrically coupled to the power plane 215. Similarly, when a power supply (not shown) is electrically coupled to a power input connector 213, the power supply is then electrically coupled to the power plane 215. As shown, power is routed from interconnect holes of the power input connector 213 to interconnect holes of components 205, 207, and 209.
Typically, each pin of the power input connector 213 is electrically connected to a power supply (not shown) having a particular voltage. For example, a pin 217 of the power input connector 213 may be connected to a 5 volt power supply. Another pin (e.g., 219) of the power input connector 213 may be connected to a different voltage and routed to other power planes (not shown) on other levels of the motherboard 202.
Conventionally, there is one power plane per level, and each power plane is used for electrically connecting to any electrical component on the motherboard 202 that requires the particular power plane's voltage level. That is, if more than one voltage level is required for the motherboard components, another layer is utilized. For example, a 5 volt power plane may lie in one layer of the motherboard and be connected to all components that require 5 volts, while a 3 volt power plane may lie in another layer of the motherboard and be connected to all components that require 3 volts.
A motherboard that employs a conventional power plane to attenuate RFI/EMI at a range of frequencies has many disadvantages. One problem with the conventional power plane is that the network of bypass capacitors coupled to the power plane only attenuate the RFI/EMI for a limited range of frequencies. That is, each time a component on the motherboard is replaced or upgraded, the network of bypass capacitors has to be reevaluated and redesigned. For example, additional capacitors may be required to bypass higher frequencies that are emitted from the new component.
This redesign of the network of bypass capacitors requires many man-hours of engineering since the engineers typically redesign the network of bypass capacitors through analysis and trial and error experiments. For instances, the engineer may need to add new capacitors to the existing capacitor network on the motherboard to filter the higher frequencies of a new electrical component, and then need to test these additional capacitors to determine whether to add more capacitors, whether to change some or all of their individual values, and whether to reposition some or all of the capacitors.
Accordingly, in view of the foregoing, there is a need for a motherboard with power planes that are designed to allow components that operate at a multiplicity of frequencies without requiring extensive redesign of the bypass capacitors on the motherboard. Additionally, there is a need for a method of implementing such a motherboard.